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Plant Physiology 146:1455-1456 (2008) © 2008 American Society of Plant Biologists On the InsidePhotosynthesis Regulation of Root Ion TransportersThe uptake rates of many ions are dependent on light conditions and fluctuate diurnally or are stimulated by an increase in light intensity. This control over root uptake systems has often been attributed to the regulatory action of sugars produced by photosynthesis and transported downward to the roots. These carbon (C) status-induced changes in root ion uptake are generally correlated with similar changes in the expression of genes encoding root ion transporters. Thus, it seems that the sugar regulation of ion transporter gene expression in the roots is a widespread mechanism, allowing the coordination of the transport of various ions with photosynthesis and the C status of the plant. Previously, it was shown in Arabidopsis (Arabidopsis thaliana) that the induction of the NRT2.1 NO3– transporter gene by sugars was dependent on C metabolism downstream of hexokinase (HXK) in glycolysis. To gain further insights into this signaling pathway and to explore more systematically the mechanisms coordinating root nutrient uptake with photosynthesis, Lejay et al. (pp. 2036–2053) studied the regulation of 19 light-/sugar-induced ion transporter genes. Various combinations of sugar, sugar analogs, light, and CO2 treatments provided evidence that these genes are not regulated by a common mechanism, and unraveled at least four different signaling pathways involved: regulation by light per se, by HXK-dependent sugar sensing, and by sugar sensing upstream or downstream HXK, respectively. Another major finding was that an inhibitor of phosphogluconate dehydrogenase almost completely prevented induction of NRT2.1 and NRT1.1 by sucrose, indicating that that the metabolism of glucose-6-P by the oxidative pentose phosphate pathway is required for generating the sugar signal. Out of the 19 genes investigated, most of those belonging to the NO3–, NH4+, and SO42– transporter families were regulated in a manner similar to NRT2.1 and NRT1.1. These data suggest that a yet-unidentified oxidative pentose phosphate pathway-dependent sugar sensing pathway governs the regulation of root nitrogen and sulfur acquisition by the C status of the plant, presumably coordinating the availability of these three elements for amino acid synthesis. Glycine Decarboxylase Subunit Gene from a C3-C4 Intermediate Species Net photosynthetic CO2 assimilation in C3 plants is reduced by photorespiration. The mitochondrial multienzyme complex glycine decarboxylase (GDC) plays a key role in photorespiration. GDC is composed of four subunits (P, H, L, and T) with the P-subunit (GLDP) serving as the actual decarboxylating unit. GDC is present in all photosynthetic cells of C3 plant leaves, but strictly confined to the bundle-sheath cells of C4 species. Conceivably, the specific expression of GLDP in the bundle-sheath cells may have constituted a biochemical starting point for the evolution of C4 photosynthesis. Plant species possessing a C3-C4 intermediate type of photosynthesis are of special interest for studying the evolution of C4-characteristic traits. As in C4 plants, functional GDC occurs only in the bundle-sheath cells of the leaves of C3-C4 intermediate plants. The genus Flaveria of the Asteraceae is a well-established experimental system for investigating the evolution of C4 characteristic traits. This genus includes both C3 and C4 species, but also a large number of C3-C4 intermediate species. To understand the molecular mechanisms responsible for restricting GLDP expression to bundle-sheath cells, Engelmann et al. (pp. 1773–1785) performed a functional analysis of a GLDP promoter from the C4 species Flaveria trinervia (Fig. 1 ). The activity of this GLDP promoter was analyzed by means of reporter gene in transgenic Flaveria bidentis (C4) and Arabidopsis (C3). Similar expression patterns were observed in both species, indicating that a mechanism for bundle-sheath-specific expression is also present in C3 species. Arabidopsis was subsequently used as a heterologous system for testing a series of promoter deletions to identify C4-characteristic regulatory elements within the GLDP promoter. These analyses resulted in the identification of two notable regions within the GLDP promoter, one conferring repression of gene expression in mesophyll cells and one functioning as a general transcriptional enhancer.
Herbivore-Induced Callose Deposition The brown planthopper (Nilaparvata lugens; BPH) is an insect that feeds on the leaf sheath of rice (Oryza sativa) plants, ingesting nutrients from the phloem. BPHs frequently cause widespread destruction of rice crops and heavy losses of yield. Feeding by numerous BPHs on a single plant results in the susceptible plants yellowing, browning, and drying. In this issue, Hao et al. (pp. 1810–1820) help shed light on an important mechanism involved in rice resistance to BPHs: callose deposition. They used a susceptible rice plant variety (TN1) as a control, and first studied the feeding behavior of the BPH on rice plants carrying the BPH-resistance genes Bph14 and Bph15. They report that feeding was often interrupted on resistant plants. Tests with [14C]sucrose showed that insects ingested much less phloem sap from the resistant than the susceptible plants. To investigate the mechanisms that prevent BPHs from continuously ingesting phloem sap from resistant rice plants, the leaf sheaths of BPH-infested and BPH-free resistant and susceptible plants were examined histologically and by real-time PCR. BPH feeding induced callose deposition in the sieve tubes at the point where the stylet was inserted and up-regulated callose synthase genes in all plants. The compact callose remained intact in the resistant plants, but genes encoding β-1,3-glucanases were activated in susceptible plants, causing unplugging of the sieve tube occlusions. Protein Complexes and Starch Synthesis in Grains
Starch is produced inside plastids and represents a major storage product of many of the seeds and storage organs produced for human consumption and industrial applications. The starch granule is a complex structure with a hierarchical order, allowing efficient packing of large amounts of glucose into a water-insoluble form. Starch granules are composed of two distinct types of glucose polymer: amylose and amylopectin. Amylose comprises largely unbranched Protein Diffusion in Grana Thylakoids Thylakoid membranes of higher plants have an intricate structure and are laterally segregated into distinct regions known as the grana and the stroma lamellae. PSII and its associated light-harvesting complex II are concentrated in the grana, while PSI and the ATPase are excluded from the grana and located instead in the stroma lamellae or at the grana margins. Thylakoid membranes in vivo, however, are not static structures. It has long been known that some protein complexes can diffuse between the grana and the stroma lamellae, and that this movement is important for various processes, including membrane biogenesis, regulation of light harvesting, and turnover and repair of the photosynthetic complexes. Grana thylakoids are among the most crowded membranes in nature: 70% to 80% of the membrane area is occupied by proteins. How can efficient protein diffusion take place in this densely packed membrane? In this issue, Kirchhoff et al. (pp. 1571–1578) report on their use of fluorescence recovery after photobleaching to probe the mobility of chlorophyll-protein complexes in isolated grana membranes from spinach (Spinacia oleracea). The authors were able to use the native fluorescence from the photosynthetic pigments to visualize protein movement, obviating the need for artificial fluorescent tags. They show that about 75% of fluorophores are immobile within a measuring period of 9 min, and suggest that this immobility is due to a protein network covering a whole grana disc. However, the remaining fraction is surprisingly mobile, which suggests that it is associated with mobile proteins that exchange between the grana and stroma lamellae within a few seconds. This contrasts sharply with computer simulations suggesting an escape time of about 1 h. Manipulation of the protein-lipid ratio and the ionic strength of the buffer reveals the roles of macromolecular crowding and protein-protein interactions in restricting the mobility of grana proteins. Mimicking the Natural Mechanical Impedance of Soil In nature, plant roots must undergo morphological changes as they navigate past obstacles in the soil. Root cap cells are the apparent sensors of touch in roots, but much remains to be learned about the signaling systems involved. One of the major barriers to understanding root growth through soil is mimicking natural soil conditions in experimental assays. In an effort to understand the effect of continuous mechanical impedance on the root morphology of Arabidopsis, Okamoto et al. (pp. 1651–1662) have developed a new growing system that provides continuous mechanical stimulation to root tips. In this growing system, Arabidopsis seedlings are grown on dialysis membrane-covered agar plates, and placed vertically (plates on edge) or horizontally (plates lying flat). The presence of the dialysis membrane prevents the root from penetrating inside the agar. Using well-characterized Arabidopsis mutants and gene expression analyses, the authors used this new technique to examine the role of ethylene in regulating the growth and development of roots. Their results suggest that it is enhanced responsiveness to ethylene rather than enhanced ethylene synthesis that plays a primary role in changing root morphology in response to mechanical impedance.
Division of Health Professions and Natural Sciences FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900255 Related articles in Plant Physiol.:
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-(1
4)-linked glucan chains and does not appear to participate in the formation of the ordered part of the matrix. Amylopectin is a branched glucan polymer typically comprising between 65% and 85% of the starch granule mass, and is produced by the formation of